The circulating catecholamine adrenaline effectively suppressed human natural killer cell cytotoxicity (NKCC) when added to mixtures of effector lymphocytes and 51Cr-labelled target cells in a 4-hour 51Cr release assay in vitro. The effect was mimicked by the beta 2-receptor agonist terbutaline but not by the beta 1-receptor agonist prenalterol or the alpha 1/alpha 2-receptor agonist clonidine. Adrenaline-induced NKCC suppression was completely and potently antagonized by the mixed beta 1/beta 2-receptor antagonist propranolol and the selective beta 2-receptor antagonist ICI 118,551 but not by the beta 1-selective antagonist metoprolol. By comparing the adrenaline sensitivity of high-density (HD) and low-density (LD) lymphocytes, fractionated by Percoll density gradient centrifugation, we found that HD cells appeared more sensitive to adrenaline-induced suppression than LD cells. In both types of effector cells, adrenaline significantly suppressed NKCC at a final concentration of 10(-11) M. Pretreatment of LD effector cells with IFN-alpha reduced the NKCC suppression by subsequent adrenaline treatment. Pretreatment with recombinant IL-2 virtually abolished the response to adrenaline. This effect was noted also when IL-2 and adrenaline were incubated simultaneously during the 4-hour 51Cr release assay. Our data suggest a role for adrenaline, via lymphocyte beta 2-receptor activation, in the regulation of natural killer cells.